1. Field of the Invention
The present invention relates to an electrochemical sensor, which has a two-part diffusion barrier, wherein a first part of the barrier forms a labyrinth with a second part of the barrier said barrier located between the measuring and counter electrode. This diffusion barrier is particularly useful in a lead-free electrochemical oxygen sensor.
2.Description of the Related Art
The market standard for oxygen sensors is a simple two-electrode sensor, in which the cathode is an expendable block of lead. The service life of these sensors depends firstly on the quantity of lead they contain and secondly on the mass flow control of oxygen to the measuring electrode. The service life is typically between one and three years. Apart from the fixed service life, another disadvantage of this type of sensor is the use of lead because of its potentially hazardous nature. This has resulted in numerous attempts to develop lead-free oxygen sensors. Sensors that use metals other than lead in a consumptive reaction (e.g. modified zinc-air batteries) must be explicitly excluded at this point, as must those that work with metal oxide electrolytes at high temperatures (e.g. lambda sondes) because the invention does neither cover a consumptive sensor nor a sensor that needs high operation temperatures (high means above 120° C.). For some time now, the companies Dräger and RAE Systems have had oxygen sensors on the market that function at ambient temperatures (−40° C. to 60° C.) according to the oxygen pump principle. In these sensors, oxygen is reduced to O2− (which further reacts to form water) at the measuring electrode (ME) and at the counter electrode (CE) O2− (from water) is oxidised to form oxygen gas. As a result the mass balance is set. This kind of sensor, however, needs a third electrode, a reference electrode (RE), against which the measuring electrode potential is maintained within a range of −300 to −800 mV. This sensor working principle is documented in numerous patents and patent applications. One of the first patents is U.S. Pat. No. 3,328,277A from the company Honeywell in 1964, in which a lead-free oxygen sensor is operated with a scavenger electrode. Further descriptions of measuring cells, which can also be used to measure gaseous oxygen, come from Dräger in the 1990's (DE 4231256 C2 and DE 1962293 C1). These firstly describe the use of different metals in the measuring electrodes and secondly identify a platinum-oxygen electrode as the stable reference electrode. The range of metals used for oxygen reduction can be extended to include other platinum metals, such as iridium, for example. However, the use of a Pt air-oxygen electrode as a reference electrode was suggested in the textbook Elektrochemische Kinetik written by Prof. Dr. K. Vetter in the 1960's (K. Vetter, Elektrochemische Kinetik, Springer Verlag, 1961), in which the author described O2 reduction on platinum surfaces. A Nernst correlation between the O2 partial pressure and the reduction potential, which patent DE 4231256 C2 wants to avoid, can therefore be excluded as improbable for standard customary electrodes.
More recent patents and patent applications deal with finding a solution to the central problems involved in constructing a lead-free O2 sensor, namely, maintaining an oxygen concentration gradient between the ME and the CE and also removing the O2 gas produced at the CE (to prevent back diffusion of O2 to the ME). The Dräger patent DE 19726453 C2 describes how the ME is protected by a fourth electrode from back diffusion of dissolved O2 gas in the electrolyte. Patent DE 19845318 C2, which follows a similar line, aims to achieve this effect using sintered electrodes. U.S. Pat. No. 6,666,963 B1 from Industrial Scientific claims that gases occurring in the sensor are removed via a pressure-compensating system. In this case, the distance required between the ME and the CE is obtained by locating the electrodes at opposite ends of the sensor.
The latest direction taught in the patent literature involves increasing the robustness of the sensors. This firstly increases their service life and breadth of application and, secondly, opens up the possibility of miniaturizing gas sensors while maintaining or improving their performance. Published patent application DE 102004037312 A1 from Drager describes the construction of a very flat sensor using ionic liquids as the electrolyte. Drager's patent DE 102004059280 B4 specifically describes a flat O2 gas sensor, in which the ME is protected from O2 back diffusion with a Nafion membrane and is also provided with an integrated memory chip. U.S. Pat. No. 7,258,773 B2 from RAE Systems works with Nafion as the solid electrolyte, to ensure there are no leaks. Protection from back-diffusion of gas to the ME occurs here too. The construction of oxygen sensors with Nafion membranes is clearly taught in Y. Osada, D.E. DeRossi, Polymer Sensors and Actuators, Springer (2000); original source: H.Q. Yan, J.T Lu (1989) Sensors and Actuators 19:33.
A more recent sensor is presented in published patent application WO 2007/115801 A1 from MST-Technology. Here, the ME and CE are on one level in the sensor. The ME is in contact with the outside world through a gas diffusion barrier; the CE is characterised by a plurality of openings designed to remove the oxygen gas generated. The ME is surrounded by a barrier placed concentrically around it. This concentric barrier is used to increase the distance between the ME and CE, which is necessary in order to create an O2 concentration gradient. The back-diffusion of O2 into the solution is thereby prevented.
One disadvantage of this simple, concentric barrier, however, is that a large number of components is required, since the ME and CE cannot be combined in a single component. The need for several components increases the probability of manufacturing errors and complicates the structure. Furthermore, only a relatively small increase in distance is possible with this simple, concentric barrier. In order to guarantee a stable gradient, the diffusion distance between the ME and CE must be of a certain minimum length.